This invention relates to methods of producing cross-linked polymeric extruded articles wherein a peroxide is employed as the cross-linking agent. In an important specific aspect, to which detailed reference will be made herein, the invention is directed to the production of cross-linked polymeric extruded coatings on electrical conductor cable.
Electrical conductor cable (the term "cable" being used herein to designate a single-strand wire as well as multistrand cable) is commonly provided with an insulating coating of an ethylene-containing polymer such as polyethylene, which must usually be cross-linked to impart required physical properties to the coating. In a conventional procedure for producing such coatings, a low-density polyethylene is blended with a minor proportion of a peroxide (e.g. dicumyl peroxide) capable of generating free radical sites in the polymer, and extruded (by means of an extruder having a feed screw) to form a continuous coating layer on a conductor core, which layer is then heated to activate the peroxide to effect cross-linking. Typically, the cable-coating composition includes, in addition to the polyethylene and peroxide, minor proportions of other ingredients including inert particulate filler materials (e.g. carbon black) and anti-oxidants.
Incorporation of substances such as filler materials in ethylene-containing polymer ordinarily requires high shear mixing; hence the filler and other additives cannot simply be blended with the polyethylene in the extruder, the extruder screw being incapable of providing the requisite mixing action. As described in U.S. Pat. No. 4,161,419 (the disclosure of which is incorporated herein by this reference), however, the filler material and other coating additives can be mixed with a suitable polymeric vehicle comprising an ethylene propylene rubber (and, typically, some polyethylene) to form a masterbatch for subsequent blending with further quantities of polyethylene to constitute an extrudable coating composition. High shear mixing, e.g. in a Banbury mixer, is required only to prepare the masterbatch; the blending of the masterbatch with the further quantities of polyethylene can satisfactorily be effected in the extruder, viz. by the action of the extruder screw, incident to the operation of extruding the blended material as a coating on the surface of an advancing cable. Advantageous savings in mixing costs are realized by this procedure, as compared with the alternative of subjecting the entire coating composition to high shear mixing.
Since producers of coated cable often do not have the specialized and very expensive equipment required for high shear mixing, they commonly obtain the above-described masterbatch from a supplier in already-mixed, pelletized form. That is to say, the supplier ships the masterbatch pellets to the cable producer, who stores them at room temperature for use as needed. Polyethylene, also in the form of pellets, is similarly purchased and stored by the cable producer. For the coating operation, a minor proportion of masterbatch pellets and a major proportion of polyethylene pellets are commingled and charged (i.e. still in discrete pellet form) to the extruder, where the charge is heated and worked into a substantially homogeneous, fluid, extrudable mass.
The peroxide cross-linking agent employed in the coating composition must be blended with polymeric material ahead of the extruder because unmixed peroxide, if introduced to an extruder screw, tends to lubricate the screw surfaces and thus to interfere with proper pumping action of the screw. In operations of the type described above, it has heretofore been customary for the masterbatch supplier to incorporate the peroxide in the masterbatch, so that the pellets of masterbatch as shipped to and stored by the cable producer already contain the peroxide required to effectuate cross-linking of the extruder coating. This practice, however, is attended with various difficulties. Production of the masterbatch in such case requires two mixing passes (the first to blend the polymer and filler, the second to add the peroxide), and thus adds to manufacturing costs, since the high-shear mixing required for the first pass develops high temperatures that would cause premature cross-linking if the peroxide were then present. Furthermore, under conditions of ambient temperature transport and storage of the masterbatch pellets, the peroxide bleeds out of the pellets, leading to such problems as stopped-up filters, sensor malfunction, dust buildup, and pellet agglomeration when moving or handling of the pellets is attempted. Alternative procedures, e.g. utilizing a ribbon or other blender to disperse peroxide for absorption in preformed ethylene-containing polymer pellets, and thereby to provide a supply of pellets incorporating the peroxide for shipment or storage and subsequent use, have not overcome these problems and generally are restricted to relatively low proportions of peroxide.